The present invention relates to the production of hollow ceramic membranes by electrophoretic deposition. In particular, the present invention relates to the production of small cross-sectional area hollow ceramic membranes by electrophoretic deposition.
It is well known to deposit coatings of material by electrophoretic deposition (“EPD”). EPD is a combination of electrophoresis and deposition. Electrophoresis is the movement of charged particles in an electric field. Deposition is the coagulation of particles into a mass.
In U.S. Pat. No. 5,580,835 to Dalzell et al., a process for creating ceramic fibers by EPD is described. The ceramic fibers produced by this process are fully dense, non-porous fibers. The described EPD process uses a colloidal metal hydrate from an aqueous sol where the metal hydroxide particle size is in the range of about 15 nm. The sols are produced by hydrolysis and peptization of an organometallic compound in an aqueous medium. The resulting ceramic fiber is non-porous and fully dense as a result of the small particle size of the sol and the sintering process. Because the sol is aqueous, hydrogen evolution is unavoidable and steps must be taken to minimize hydrogen evolution and to permit hydrogen to escape such that it does not embed in the deposited material. One means of doing so disclosed in this patent is to use a low potential and to continuously move the fiber during the deposition process.
As is apparent in the Dalzell et al. Patent, it is conventionally believed that in order to achieve uniform deposition, only ceramic particles of submicron size may be used in an EPD process. As a result, the resulting ceramic materials, after sintering, are not porous.
It is desirable for certain applications to produce a porous hollow ceramic fibre or membrane. Such fibres may be produced by extruding a mixture of ceramic powder and polymeric binder as disclosed in U.S. Pat. No. 5,707,584. The extruded tube or fibre may then be heat treated to remove the polymeric binder leaving a porous ceramic matrix. The porous ceramic matrix may then be coated by dipping in sols, drying and sintering to add thin layers to the microporous matrix. These are difficult and costly methods. It would be advantageous to have an alternative method of producing porous ceramic fibres or tubes or hollow ceramic membranes.
Therefore, there is a need in the art for a method of producing porous ceramic fibres or hollow ceramic membranes by electrophoretic deposition.